MagicaVoxel - A free lightweight GPU-based voxel art editor and interactive path tracing renderer.
Blender - Blender is the open source, cross platform suite of tools for 3D creation.
SpriteStack - 3D pixelart editor based on sprite stacking technique.
Sculptris - Sculptris: Enter a world of digital art without barriers.
Cubik Studio - Model in a unique cubic style. Start modelization with boxes. Move, rotate and scale cuboids.
Misfit Model 3D - Misfit Model 3D is a 3D model editor that basically operates with triangle-based models.